In recent years, the use of image forming apparatuses such as a printer has been shifting increasingly from office to personal purposes, and there is a growing demand for technologies that can achieve not only maintenance-free use but also a small size, a high speed, and high image quality for those apparatuses. Under such circumstances, e.g., a cleanerless process, a tandem color process, low-temperature fixing, and oilless fixing are required along with better maintainability and less ozone emission. The cleanerless process allows residual toner from the transfer to be recycled for development without cleaning in an electrophotographic system. In the tandem color process, image forming units for different colors are arranged to form color images simultaneously, so that the color images can be outputted at high speed. The low-temperature fixing contributes to high-speed printing and energy saving. The oilless fixing can provide clear color prints by preventing a so-called offset phenomenon, in which toner adheres to the surface of a fixing roller of a fixing device, without using any release oil (fixing oil). It is desired that all of these functions be performed at the same time. Therefore, in the development of the above technologies, improvements in the toner characteristics as well as the image forming process are important factors.
For example, while toner particles should be small enough to provide higher resolution and higher image quality of prints, in a fixing process for color images of a color printer, it is also necessary that each color of toner be melted and mixed sufficiently to increase the transmittance. In this case, a melt failure of the toner may cause light scattering on the surface or the inside of the toner image (i.e., the image composed of toner), and the original color of the toner pigment is affected. Moreover, light does not reach the lower layer of the superimposed layers of different colors of toner, resulting in poor color reproduction. Therefore, in addition to a reduction in particle size, the toner should be adapted to low-temperature fixing and have a sufficient melting property and transmittance high enough not to reduce the original color. In particular, the need for light transmittance of an over head projector (OHP) film is increasing with an increase in opportunities to give a color presentation.
However, when the toner with a sufficient melting property is used, a high-temperature offset (hot offset) phenomenon in which the toner adheres to the surface of a fixing roller is likely to occur. To suppress such an offset phenomenon, a large amount of a release agent such as oil (fixing oil) should be applied to the fixing roller, which makes the handling and configuration of the fixing device more complicated. Therefore, oilless fixing (no oil is used for fixing) is required to provide a compact, maintenance-free, and low-cost apparatus. Moreover, the toner needs to satisfy both the aggregation performance that prevents high-temperature offset and coagulation of the toner during storage and the melting performance that allows the toner to melt at low temperatures for low-temperature fixing and improves the transmittance or the like.
The toner generally includes a resin component as a binder, a coloring component of a pigment or dye (i.e., a coloring additive), and any other additives such as a plasticizer, a charge control agent, and if necessary, a release agent (wax). As the resin component, a natural or synthetic resin may be used alone or in combination. After the above additives are pre-mixed in an appropriate ratio, the components are heated, kneaded, and thermally melted. Then, it is pulverized by an air stream collision board system and classified as fine powder, thus producing a toner base. The toner base also may be produced by chemical polymerization instead of the kneading and pulverizing processes.
Subsequently, an additive such as hydrophobic silica is added to the toner base, so that the toner is completed. A single-component developer includes only the toner, while a two-component developer is obtained by mixing this toner and a carrier composed of magnetic particles.
At present, various methods are considered to produce the toner base particles with a small particle size. Even with pulverization and classification of the conventional kneading and pulverizing processes, the actual particle size can be reduced to only about 8 μm in view of the economic and performance conditions. Therefore, various ways of polymerization different from the kneading and pulverizing processes have been studied further as a method for producing a toner base.
For example, a toner base may be produced by suspension polymerization. In this method, however, the particle size distribution of the toner base is no better than that of the toner base produced by the kneading and pulverizing processes, and in many cases further classification is necessary. Moreover, since the toner base obtained by this method is almost spherical in shape, the toner remaining on the photoconductive member of an electrophotographic apparatus does not clean successfully, and thus the reliability of the image quality is reduced.
Also, a toner base may be produced by emulsion polymerization. This method includes the following steps: preparing an aggregated particle dispersion by forming aggregated particles in a dispersion that has been obtained by dispersing at least binder resin particles (also referred to as first binder resin particles when they are distinguished from second binder resin particles, as will be described later) in an aqueous medium containing a surface-active agent; adding a second resin particle dispersion in which second binder resin particles are dispersed to the aggregated particle dispersion; and heating the resultant mixture so that the second binder resin particles are fused with the aggregated particles (also referred to as core particles) to form a resin fused layer.
To achieve the oilless fixing with the toner as described above, the configuration in which a release agent (wax) is added to a binder resin with a sharp melting property, i.e., a sufficient melting property is being put to practical use.
However, such a toner is very prone to a transfer failure or toner image disturbance during transfer because of its strong cohesiveness. Therefore, it is difficult to ensure the compatibility between transfer and fixing. When a toner base is produced by adding a release agent (wax) to the resin with a low softening property during melting and kneading, problems such as low flowability of the toner, transfer failures including transfer voids, and so-called toner filming in which the toner components adhere to a photoconductive member arise as the amount of wax increases. Thus, there is a limit to the amount of wax that can be added. Moreover, when the toner is used as a two-component developer, a so-called spent phenomenon in which a low-melting component of the toner adheres to the surface of a carrier to form a toner film is likely to occur due to heat generated by mechanical collision or friction between the particles of the toner and the carrier or between the particles and the developing unit. This decreases the charging ability of the carrier for the toner and interferes with a longer life of the two-component developer.
To deal with the above problems, Patent Document 1 discloses a coating carrier for positively charged toner that is obtained by introducing a fluorine-substituted alkyl group into a silicone resin of the coating layer. Patent Document 2 discloses a coating carrier that includes conductive carbon and a cross-linked fluorine modified silicone resin. This coating carrier is considered to have high development ability in a high-speed process and maintain the development ability for a long time. While taking advantage of superior charging characteristics of the silicone resin, these carriers use the fluorine-substituted alkyl group to impart properties such as slidability, releasability and repellency, to increase resistance to wearing, peeling or cracking, and further to prevent spent.
In the emulsion polymerization method, Patent Document 3 discloses a process of preparing a liquid mixture by mixing at least a resin particle dispersion in which binder resin particles are dispersed in a surface-active agent having a polarity and a colorant particle dispersion in which colorant particles are dispersed in a surface-active agent having a polarity. The surface-active agents included in the liquid mixture have the same polarity, so that a toner for electrostatic charge image development with high reliability and excellent charge and color development properties can be produced in a simple and easy manner.
Patent Document 4 discloses that the release agent includes at least one type of ester composed of at least one selected from higher alcohol having a carbon number of 12 to 30 and higher fatty acid having a carbon number of 12 to 30, and the binder resin particles include at least two types of binder resin particles with different molecular weights. This can provide a toner with an excellent fixing property, color development property, transparency, and color mixing property.
Patent Document 5 discloses that the molecular weight distribution of a resin component has a peak or shoulder in the range of at least 1,500 to 20,000 and 50,000 to 500,000, Mw/Mn is 1.2 to 4.0 in the molecular weight distribution (ML) derived from a peak or shoulder on the lower molecular weight side, and Mw/Mn is 2.0 to 30.0 in the molecular weight distribution (MH) derived from a peak or shoulder on the higher molecular weight side. Patent Document 5 also discloses the addition of an olefin wax such as polypropylene or polyethylene, a modified material thereof, a natural wax such as carnauba wax or rice wax, an amide wax such as fatty acid bisamide, etc. This configuration provides high offset resistance during heat fixing, so that high-quality visible images can be formed stably for a long time.
In the example of Patent Document 5, a manufacturing method is described that includes salting-out/fusing a mixture of a latex 1 in which low molecular weight resin particles are dispersed, a latex 5 in which high molecular weight resin particles are dispersed, a colorant dispersion 1, and a wax emulsion (polypropylene emulsion).
Patent Document 6 discloses a toner obtained by salting-out/fusion, in which the resin includes at least a low molecular weight component having a peak or shoulder in the range of 1,500 to 20,000 and a high molecular weight component having a peak or shoulder in the range of 50,000 to 500,000 of a GPC molecular weight distribution, and the release agent has a peak in the range of 70° C. to 100° C. based on DSC. The toner has an excellent cleaning property and charging stability, so that high-quality images can be formed for a long time. In the example of Patent Document 6, a manufacturing method is described that includes stirring a low molecular weight resin particle dispersion latex 1), a high molecular weight resin particle dispersion (latex 2), a colorant particle dispersion 1, and a release agent particle dispersion 1 and salting-out/fusing the mixture.
Patent Document 7 discloses a toner obtained in the following manner. Resin particles (A) having a weight-average molecular weight (MwA) of 15,000 to 500,000 and colorant particles are salted out/fused to form colored particles (core particles), and then resin particles (B) having a predetermined molecular weight are fused with the surface of the individual colored particles to form a resin layer (shell) by salting-out/fusion. Since the amount of a colorant present on the particle surface is small, high charging and developing performance of the toner is not likely to be affected by the operating environment.
However, in the conventional configurations of Patent Documents 1 and 2, when a toner including a release agent such as wax is used in a two-component developer, the coating layer of the carrier is not sufficient to suppress wearing, peeling or cracking. Moreover, when a negatively charged toner is used, the charge amount of the toner is too low, while an oppositely charged toner (positively charged toner) is generated in large quantity, causing fog, toner scattering, or the like. Thus, the two-component developer is not suitable for practical use.
In the conventional configurations of Patent Documents 3 and 4, a release agent such as wax is added during the production of a toner base with a polymerization method, and thus the toner can achieve oilless fixing, reduce fog in the development, and improve the transfer efficiency. However, it is difficult to incorporate the wax uniformly into the aggregated particles. Therefore, the dispersibility of the wax is reduced, and the toner images melted during fixing are prone to have a dull color.
Further, when these aggregated particles are used as core particles, and the second binder resin particles are attached and melted on their surfaces to form a rein fused layer, the adhesion of the second binder resin particles does not proceed because the individual aggregated particles cannot incorporate the wax uniformly, and the wax dispersibility is low. Otherwise, the second binder resin particles that once adhered to the aggregated particles may be separated therefrom due to the releasing action of the wax present on the surfaces of the aggregated particles, and thus may remain suspended in the aqueous medium. If the second binder resin particles are fused forcibly with the aggregated particles by controlling the heating conditions, the particles themselves tend to be coarser.
The dispersion of the release agent (wax), depending on its polarity or thermal properties such as a melting point, may have a considerable effect on the aggregation of the particles. Moreover, a specified wax should be added in large quantity to achieve the oilless fixing. When the toner base particles are produced by an aggregation reaction in the medium that contains at least a certain amount of wax, the particle size is likely to increase with heat treatment time.
In particular, when a plurality of waxes with different melting points or compositions are used to achieve both low-temperature fixability and high-temperature offset resistance, thereby broadening the fixable temperature range, a low melting point wax starts to melt and is aggregated with a colorant or partially melted resin particles as the temperature of an aqueous medium is raised to produce aggregated particles. However, a high melting point wax does not start to melt at this stage and is still present in the aqueous medium without being melted, and thus is not involved in the aggregation reaction. Therefore, some wax is melted and aggregated continuously, while other wax is not aggregated. Consequently, the wax dispersion may vary among the aggregated particles produced, and there may be some cases where the surfaces of the aggregated particles are rich in wax, the particle size of the aggregated particles is increased, or the particle size distribution becomes broader.
In the conventional toner, the use of a release agent such as wax, particularly a plurality of waxes with different melting points or compositions prevents uniform mixing and aggregation of the particles including the binder resin particles and the colorant particles in the aqueous medium during manufacture. Thus, some wax is not aggregated but suspended in the aqueous medium, causing the molten aggregated particles that serve as toner base particles to be coarser. As a result, it is difficult to produce toner base particles having a small uniform particle size. Moreover, when the conventional toner is mixed with a carrier as a two-component developer, the two-component developer deteriorates easily due to a so-called spent phenomenon in which a component such as wax adheres to the surface of the carrier.
Patent Document 1: Japanese Patent No. 2801507
Patent Document 2: JP 2002-23429 A
Patent Document 3: JP 10 (1998)-198070 A
Patent Document 4: Japanese Patent No. 3399294
Patent Document 5: JP 2001-154405 A
Patent Document 6: JP 2001-134017 A
Patent Document 7: JP 2002-116574 A